Comparison of instrumented and stand-alone lateral lumbar interbody fusion for lumbar degenerative disease: a systematic review and meta-analysis

Background Both instrumented and stand-alone lateral lumbar interbody fusion (LLIF) have been widely used to treat lumbar degenerative disease. However, it remains controversial as whether posterior internal fixation is required when LLIF is performed. This meta-analysis aims to compare the radiographic and clinical results between instrumented and stand-alone LLIF. Methods PubMed, EMBASE and Cochrane Collaboration Library up to March 2023 were searched for studies that compared instrumented and stand-alone LLIF in the treatment of lumbar degenerative disease. The following outcomes were extracted for comparison: interbody fusion rate, cage subsidence rate, reoperation rate, restoration of disc height, segmental lordosis, lumbar lordosis, visual analog scale (VAS) scores of low-back and leg pain and Oswestry Disability Index (ODI) scores. Results 13 studies involving 1063 patients were included. The pooled results showed that instrumented LLIF had higher fusion rate (OR 2.09; 95% CI 1.16–3.75; P = 0.01), lower cage subsidence (OR 0.50; 95% CI 0.37–0.68; P < 0.001) and reoperation rate (OR 0.28; 95% CI 0.10–0.79; P = 0.02), and more restoration of disc height (MD 0.85; 95% CI 0.18–1.53; P = 0.01) than stand-alone LLIF. The ODI and VAS scores were similar between instrumented and stand-alone LLIF at the last follow-up. Conclusions Based on this meta-analysis, instrumented LLIF is associated with higher rate of fusion, lower rate of cage subsidence and reoperation, and more restoration of disc height than stand-alone LLIF. For patients with high risk factors of cage subsidence, instrumented LLIF should be applied to reduce postoperative complications. Supplementary Information The online version contains supplementary material available at 10.1186/s12891-024-07214-6.


Introduction
Lumbar degenerative disease is a common condition treated by spine surgeons, which can induce dysfunction and decrease of quality of life.Lateral lumbar interbody fusion (LLIF), including extreme/direct lateral interbody fusion [1,2], and oblique lateral interbody fusion (OLIF) [3], has been used to treat lumbar degenerative disease.LLIF is a minimally invasive technique with satisfactory result in indirect decompression of spinal canal and foramina.
Both instrumented and stand-alone LLIF have been widely used and proved effective in clinical work.However, it remains controversial as whether posterior internal fixation is required when LLIF is performed [4].Some reports showed that stand-alone LLIF could achieve equivalent clinical and radiological results like instrumented LLIF [5].And stand-alone LLIF is associated with short operation time, small trauma, and much more costeffective [6,7].On the other hand, some authors argue that instrumented LLIF has lower rate of postoperative complications [8] including cage subsidence, nonunion, and reoperation.There is still a lack of evidence-based medicine to prove the clinical results between instrumented and stand-alone LLIF.Therefore, we performed a systematic review and meta-analysis to compare the efficacy between instrumented and stand-alone LLIF.

Inclusion criteria
The inclusion criteria of this meta-analysis: (1) target population: patients with lumbar degenerative disease including disc herniation, stenosis, spondylolisthesis and so on; (2) intervention: LLIF with posterior instrumentation (instrumented LLIF) versus stand-alone LLIF.Only studies comparing these two techniques were included; (3) methodological criteria: prospective or retrospective trials.Reviews, case reports and biomechanical analysis were excluded.Studies that could not provide adequate information on the mean or odds ratio were excluded.

Quality assessment
Quality of the included studies was assessed independently by two authors (X.Z. and L.Q.).The Newcastle Ottawa Quality scale [9] was used to for the assessment of prospective or retrospective studies.

Data extraction
Data extraction was performed by two authors (L.J. and L.L.) independently.General characteristics of the included studies were recorded: study design, year of publication, first author, sample size, and follow-up time.The clinical and radiographic outcomes were extracted from studies for comparison: interbody fusion rate, cage subsidence rate, reoperation rate, restoration of disc height, segmental lordosis, lumbar lordosis, visual analog scale (VAS) scores of low-back and leg pain and Oswestry Disability Index (ODI) scores.

Statistical analysis
The abstracted data were analyzed using Review Manager version 5.3 (Cochrane Collaboration).Continuous data were presented in terms of mean difference (MD) and 95% confidence interval (CI); and dichotomous data were presented in terms of odds ratio (OR) and 95% CI.Statistical heterogeneity among the studies was checked using the χ2 test.P > 0.10 or I 2 < 50% indicated that there was no significant heterogeneity, and the fixed-effects model was used.Otherwise, P < 0.10 or I 2 > 50% indicated significant heterogeneity.The random-effects model was used when the source of heterogeneity could not be found.

Quality of the Individual studies
The 13 studies included 3 prospective studies [11,17,19] and 10 retrospective studies [5, 10, 12-16, 18, 20, 21].Quality of the included studies was evaluated according to the Newcastle-Ottawa Scale.Of the 13 studies, ten were high-quality with scores 8-9, and three were moderate with a score of 7 (Table 1).Baseline characteristics of included studies were shown in Table 2.

Cage subsidence
Ten studies [5, 10-13, 15-17, 19, 20] presented the cage subsidence rate at the last follow-up.Pooled results revealed a significantly lower cage subsidence rate in the instrumented group than the stand-alone group (OR 0.50; 95% CI 0.37-0.68;P < 0.001; heterogeneity: P = 0.44, I 2 = 0%, fixed-effects model, Fig. 3).Funnel plot for the cage subsidence rate was used to assess the publication bias.As the funnel plot appeared symmetrical, no significant publication bias was found in this meta-analysis (Fig. 4).

Discussion
For years, as a minimally invasive approach, LLIF is one of the most commonly used techniques [1,[22][23][24][25].Both instrumented and stand-alone LLIF have been widely performed in clinic work [26][27][28].Some studies noted that stand-alone method could be sufficient to achieve stabilization and fusion [17,[29][30][31].However, other studies proved that stand-alone LLIF are associated with higher rate of nonunion and cage subsidence [11], which would impact the clinical outcome.Hence, we perform this meta-analysis to compare the efficacy between instrumented and stand-alone LLIF for lumbar degenerative disease.Fusion is of great importance for patients who underwent LLIF.In the previous systematic review by Manzur et al., the instrumented LLIF group had a higher fusion rate than the stand-alone group (91.0% vs. 80.4%) [32].Similarly, our meta-analysis showed the fusion rate was higher in the instrumented group than the stand-alone group (92.7% vs. 84.5%,P = 0.01).The higher fusion rate may be result from sufficient rigidity and limited range of motion provided by posterior fixation [33] .
Cage subsidence is one of the most common complications after LLIF [34][35][36][37][38][39], which is associated with factors like osteoporosis, endplate violation, and higher BMI.This meta-analysis showed the instrumented group had lower cage subsidence rate than the stand-alone group.This result can be explained by the fact that posterior instrumentation could improve the stability and distribute load across the endplate.Therefore, patients with high risk factors of cage subsidence are advised to take the instrumented LLIF.On the contrary, for patients without the risk factors of cage subsidence including osteoporosis, endplate violation, and higher BMI, standalone LLIF can be considered.This meta-analysis also showed the instrumented group had more restoration of disc height at the last follow-up, which is related to the lower cage subsidence rate.
With respect to clinical outcomes, previous review by Alvi et al. demonstrated comparable ODI and VAS scores between instrumented and stand-alone groups at the last follow-up [8], which was consistent with the results in our study.Though the stand-alone group had a higher rate of cage subsidence, most cases were low-grade subsidence and were mostly asymptomatic.However, it should be noted that only three studies were included in the comparison of ODI score, and significant heterogeneity was detected in the in the comparison of low-back VAS score.More high quality studies are needed for further evaluation.Clinical outcomes are also highly correlated with spinal alignment and spinopelvic parameters (pelvic index, pelvic tilt, sacral slope, sagittal vertical axis) [40], as well as spino-pelvic-femoral parameters such as femoral obliquity angle (FOA) and T1 pelvic angle (TPA).FOA > 10°and increased TPA were reported to be associated with worse clinical and functional outcomes [41].Spinopelvic and spino-pelvic-femoral parameters should be considered in the future meta-analysis.
There are several limitations in this meta-analysis.First, there is no randomized controlled trial included in this study.Second, the number of patients included in the meta-analysis is relatively small.Third, patients included in this meta-analysis had different lumbar degenerative Fig. 5 Forest plots of reoperation rate in instrumented and stand-alone groups Fig. 4 Funnel plot for cage subsidence rate to assess publication bias among included studies disease including degenerative disc disease, spondylolisthesis, adjacent segment disease, scoliosis and so on.All the lumbar degenerative disease were put together in this meta-analysis, which may lead to significant heterogeneity.
In summary, based on this meta-analysis, instrumented LLIF is associated with higher rate of fusion, lower rate of cage subsidence and reoperation, and more restoration of disc height than stand-alone LLIF.For patients with high risk factors of cage subsidence, instrumented LLIF should be applied to reduce postoperative complications.

Fig. 1
Fig. 1 Flow diagram depicting the literature search and selection process

Fig. 3 Fig. 2
Fig. 3 Forest plots of cage subsidence rate in instrumented and stand-alone groups

Fig. 6
Fig. 6 Forest plots of restoration of disc height (A), segmental lordosis (B), and lumbar lordosis (C) in instrumented and stand-alone groups

Table 1
Quality assessment of the studies included according to newcastle-ottawa scale

Table 2
Baseline characteristics of studies included in this meta-analysis